Development and characterisation of new fire retardant composites

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Elsevier Ireland.

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0141.3910/‘)6/$15.00

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and characterisation retardant composites B. Castes*

AEROSPATIALE,

in Northern

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of new fire

& Y. Henry

Joinr Research Center, 4bis rue dti Val d’Or. 92 152, Suresnes Cedex, France

Fire, mechanical and thermal properties of novel fire retardant composites developed in the BRITE-EURAM 4412 Programme have been assessed with typical materials currently used in aerospace, such as epoxy based materials for structural parts and phenolic based materials for internal furnishing. These newly developed materials (neat resins and composites) present a significant improvement in their properties from 40 to 70% without loss of thermal and mechanical behaviour 0 1996 Elsevier Science Limited

1 INTRODUCTION

This conference presents the results obtained on composites. The following properties are detailed: l

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This study has been carried out in the BRITE-EURAM Programme 4412 under the Authority of EC-DG XII. The objectives have been described in the general presentation (1st general conference: ‘Development of new materials with improved fire resistance, reduced smoke and toxity’). New materials have been developed on the basis of an original concept of reactive (or telechelic) phosphorylated fire retardants, included and crosslinked in the network. More than 20 new molecules have been synthesised, six epoxy, BMI and cyanate formulations developed, and finally three formulations have been selected (two epoxies and one BMI based formulation). and relevant From this, three prepregs composites have been manufactured. This presentation gives the assessment of the final properties (mechanical, thermal and fire), the comparison with a current aeronautical composite material and the confirmation of the validity of the new concept of fire retardants.

Physical properties: density, micrography, fiber content. Thermomechanical properties: DSC (residual enthalpy of polymerisation), Tc (wet and dry by DMA). Mechanical properties: ILSS (Inter Laminar Shear Strengh), 3 point-bending, compression, tensile (with and without open hole) and CA1 (compression after impact). Fire properties: OSU test, flammability, NBS chamber (smoke and toxity) and burn through test (kerosene torch).

This characterisation has been compared and completed with other different tests carried out by 3M and the University of Torino (e.g. LOI, cone calorimeter); a good correlation has been observed. The very large improvement of the fire properties obtained confirms the validity of this new concept of reactive fire retardants (included and linked in the polymer network) and opens wide possibilities for applications in different sectors of industries and with other different processes such as RTM.

2 OBJECTIVES The main objectives of this project new materials with:

*To whom

all correspondence should be addressed at: AEROSPATIALE, Centre Commun de Recherches Louis BICriot, Departement Materiaux, 12, rue Pasteur, BP 76, 92152 Suresnes, Cedex, France.

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B. Cosres, Y. Henry

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the conservation, even thermal and mechanical T,;).

3 MATERIALS

an increase in the properties (e.g. wet

6 FIRE

OSU, BUNSEN flammability, NBS, Torch, calorimeter and LOI. (a) OSU (OHIO STATE UNIVERSITY) according to the specifications:

DEVELOPED

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Three UD carbon fiber composite materials have been developed on the basis of the three selected formulations: l

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F2: toughened epoxy/phosphine F3: untoughened BMI/phosphine F7: toughened epoxy/phosphazene

oxide oxide

A current aeronautical epoxy composite has been selected for comparison as a reference under the abbreviated code: Ep.Ref.

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5 THERMAL AND CHARACTERISATION

THERMOMECHANICAL

(a) All the systems are fully polymerised, no residual enthalpy of polymerisation has been Scanning DSC (Differential detected by Calorimetry). (b) Dry and wet T,; (after ageing for 500 h, 7O”C, 95% RH) have been determined by Torsion Mechanical DMTA, (Dynamic Analysis). of the formulations are Dry T,; values comprised between 175 and 205°C for epoxies and 250-260°C for BMI. Wet T,; values decrease by approximatively 30°C for epoxies and 25°C for BMI systems. The wet T,; values reached the initial objective. i.e. wet T,; =160-170°C for epoxies (a slightly lower value is found for F2, but it is still acceptable).

-40% -60%

for epoxy systems, for BMI systems.

The values are close to the best phenolic system used in aerospace furnishing. The weight loss has the same magnitude as the epoxy reference, but the time of HR max (-2 min) is twice as long.

All the composite types (9: 3 resins X 3 lyings) were physically characterized:

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F.

. The three developed composites are in accordance with the specifications: HR and HR 2 min ~65 kW/m’. Compared with the current epoxy reference material an important decrease in HR is observed:

PROPERTIES

by measurement of the density and fiber content: by micrographic observations (X20 up to X200) they show in some places an important porosity inside the material.

tests

The main values of heat release HR max and HR 2 min lead to the following observations:

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FAR 25. Amdt 25-72. $25853(c). App. Part IV JAR 25. 5 25853(al). App. F. Part IV

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4 PHYSICAL

TESTS

(b) BUNSEN the specifications:

flammability

tests

according

to

. FAR 25. Amdt 25-72. $25-853(a). App. F. Part I JAR 25. $25-853(a). App. F. Part I All the BUNSEN flammability values (flame extinction and length of burn) fully comply with the specification and are very close together (F2, F3 and F7). Compared with the epoxy reference an improvment of 100% is obtained. (c) NBS (National Bureau of Standard): smoke and toxicity according to FAR 25, JAR 25 and ATS lOOO-Issue 5 (or ABD 031) All the tests are in accordance with the specification: l

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smoke density ~~200 gases evolved: CO ~3.500 ppm << 100 ppm

and

others

and at the level of the epoxy reference, except for the BMI system which shows better values. No halogenated fumes are emitted during burning. (d) Torch: burnthrough of panels with a kerosene torch.

Development

and characterisation

Irradiance: 119 kW/m’-T = 1 120°C. No burnthrough was observed after 1.5 min of exposure to the kerosene torch for any material (2 mm thick samples). (e) Additional test: cone calorimeter: (3M, University of Torino). This method (close to OSU and NBS) was applied to the three materials, the epoxy reference and other references (fire retardant epoxy and phenolics systems). l

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HR cone values are ranked in the same way as the OSU HR values. shows Comparison with the epoxy reference for epoxies and an improvement of -30% -50% for BMI systems.

(f) Additional test: LOI-Limit Oxygen Index(3M, University of Torino, SNPE). This method gives the same ranking for the three formulations as for the previous methods and shows that the orientations of the fibers has a great effect on the results (horizontal >> vertical). In conclusion, the newly developed composites show an improvement for all the fire characteristics and, especially in the case of heat release values (HR), a decrease of: l

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40-50% 65-75%

for epoxy systems (F2, F7), for the BMI system (F3),

compared with ‘typical’ epoxy reference systems. All the values are in accordance with the specifications. HR values are comparable with the best phenolic systems currently used in aerospace furnishing applications. Our main objective, i.e. improvement of HR of at least 40% is reached.

7 MECHANICAL

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Five different temperatures 20, 70, 120, 150 and -55°C without ageing; Three different temperatures 20, 70 and 120°C with wet ageing (1000 h, 7O”C, 95% FIR); Room temperature after MEK immersion.

The greater

ILSS values than the

331

composites

specifications (except for F2, which has a slightly lower value). The decrease A/(2O”C ref) of ILSS at different temperatures or conditions is equivalent or better than the reference. Water and MEK uptake were also determined at the end of aging, and water uptakes are ranked as follows: F2 (0.35%) < F3 (0.45%) < F7 (0.60%) and MEK uptake is very small, (-0.02%). (b) Tensile and open hole tensile. The values including the ‘hole coefficient’ have the same magnitude as for the epoxy reference, even slightly better. (c) 3 point-bending. The same observation was seen as for tensile tests. (d) Compression. The compression values of the new formulations are good, better than the reference material and the specification values. (e) CAI. Samples were impacted at eight different energies from 6 to 30 J, examined by C-Scan (NDC) and CA1 was performed. The impact behaviour of the new formulations is good, than the epoxy reference. generally better Delaminated area and impact depth versus energy are better, CA1 is close to the reference, 7.1 Conclusion

on mechanical

properties

The test values fully comply with the specification and the reference epoxy, with a slight improvement in some cases. The newly developed materials have good mechanical properties, above the level of the performance required for aerospace applications.

CHARACTERISATION

The following mechanical properties were tested on the three developed composite materials: (a) ILSS: Interlaminar shear strength. Measurements at: l

of new fire retardant

of the three materials reference epoxy and

are the

8 MOULDING PIECES

OF REAL

AERONAUTIC

The work of this programme was completed by the manufacture of representative workpieces for the aeronautic sector, which represents a demonstration of the developed materials. The manufactured piece is a rear lateral engine hood for the ATR 42-72 planes, currently manufactured in typical epoxy/carbon tissue material. have been built with the Three pieces agreement of AEROSPATIALE, starting from the three prepregs F2, F3 and F7.

B. Castes,

332

The appearance of the workpieces is satisfactory. An ultrasonic non-destructive control (C-Scan Jet), together with other methods such as manual US, radioscopy and tape test have been performed at AEROSPATIALE. The quality of the core of the pieces is acceptable, especially good adherence at the interface between honeycomb and composite. The homogeneity and porosity are good for epoxy pieces, but not for the BMI piece (it seems that the cure cycle, with high temperatures, should be optimized for this resin in industrial conditions). In summary. as a demonstration, three representative workpieces have been manufactured with the newly developed materials. Good quality has been obtained, confirmed by ultrasonic NDC (see Fig. 1).

9 CONCLUSIONS The main targets of this research based on a new approach to fire retardant materials have been successfully reached and have led to the preparation of new materials with improved fire resistance, low toxicity and without the loss of mechanical and thermal properties. Fire properties assessed by OSU, NBS, BUNSEN, Torch, Cone, LO1 tests appear to be very good, especially (it is noticed) for the heat release HR (the most important characteristic). The following conclusions are made: l

An HR improvement of 4O-50% for the epoxy system and 6S-75% for BMI system.

Y. Henry

compared with a typical non-fire-retardanced system (914). HR values close to one of the best (fire behaviour) aerospace phenohc systems. HR values in full compliance with the specifications. No release of major halogenated toxicants and decrease of fire properties. No burnthrough by kerosene torch on 2mm thick samples. All the other tire properties show similar levels and improvements. Thermal and mechanical properties obtained are comparable with (or a little higher than) those obtained with current non-fire-retardant materials, no loss of mechanical behaviour has been observed with fire retardants. Real aerospace assemblies, rear lateral engine hoods for ATR 42-72. have been manufactured for each formulation as a demonstration and a further control shows a satisfactory quality. In summary, new materials based on phosphorus containing telechelic molecules have been successfully developed and allow the formulation to reach an increase of 40-60% of the fire retardant properties with conservation of mechanical and thermal behaviour. The originality of this new concept of reactive fire retardant is undoubtedly the main reason for this behaviour since the fire retardant molecules directly contribute to the cohesion of the network by crosslinking. This could still lead to major and further development and improvement by using this new concept in acting not only on modified hardeners but also on the basic matrices and other necessary additives. Following this, optimisation of the processing parameters could bc expected to produce fire retardant formulations capable of RTM technology. This further development will produce a major improvement in security for transport vehicles. in allowing for the first time the replacement of current phenolic systems by new improved systems with higher performance packages. ACKNOWLEDGEMENTS

Fig. 1. Demonstration

piece: rear lateral ATR 42-72.

engine

hood

for

The members of the consortium wish to thank the commission of the European Union. DG XII, for the financial support of this project through the BRITE-EURAM Programme: Project BE 4412: Programme BREU CT 91-0466.